1. Field of the Invention
The present invention relates to a fuel cell system having a reforming reactor for reforming hydrocarbon fuel to hydrogen-rich gas, and in particular, to those having a reforming reactor which has superior starting characteristics.
2. Description of the Related Art
Japanese Unexamined Patent Application, First Publication Nos. Hei 5-290865, Hei 7-192742, and Hei 7-240223 disclose examples of fuel cell systems, in which hydrocarbon fuel such as methanol is reformed to hydrogen-rich fuel gas by using a reforming reactor, and the reformed fuel gas and an oxidizing gas such as air are supplied to the fuel cells, thereby generating power.
Such a fuel cell system must be warmed up so as to start the system. An example of the warming-up method will be explained with reference to FIG. 5.
FIG. 5 shows the general structure of a fuel cell system which can be warmed up at the starting time. In the figure, reference numeral 70 indicates a reforming reactor which comprises an autothermal reformer 71 for reacting evaporated fuel and air (for reforming) with each other and obtaining reformed hydrogen-rich gas; a first heat exchanger 72 for decreasing the temperature of the reformed gas which is generated by the reformer 71; a CO remover 73 for oxidizing the CO included in the reformed gas so as to generate CO2; and a second heat exchanger 74 for decreasing the temperature of the reformed gas from which the CO has been removed to a certain temperature so that the reformed gas can be supplied to a fuel cell stack 60.
The reformed gas generated by the reforming reactor 70 is supplied as fuel gas to an anode of the fuel cell stack 60. The supplied gas reacts with air which is supplied to a cathode and which functions as an oxidizing gas, thereby generating power. If the CO content of the reformed gas supplied to the fuel cell stack 60 is high, the anode of the fuel cell stack 60 is subjected to CO poisoning, and the output power is decreased. Therefore, CO is removed from the reformed gas by using the CO remover 73.
After the power generation, the fuel gas which was supplied to the anode is discharged as “off gas” from the fuel cell stack 60, and the discharged gas is transferred to a catalytic combustor 61. In this catalytic combustor 61, the hydrogen which remains in the off gas is combusted, so that the temperature of the off gas increases and this heated off gas is supplied to an evaporator 62. This evaporator 62 is provided for heating and evaporating the original fuel, which is supplied to the reforming reactor 70, by using the waste heat of the off gas, so that evaporated fuel is obtained. The evaporated fuel is supplied to the reformer 71 of the reforming reactor 70 together with the heated air used for reforming, and the off gas is discharged from the evaporator 62.
A starting burner 75 is provided at the reformer 71 of the reforming reactor 70. The original fuel and air can also be supplied to the starting burner 75 and they are supplied only when the system is warmed up at the system start, so that the burner 75 is ignited. The high-temperature combustion gas generated by the burner 75 is transferred to the reformer 71, and the air and original fuel, which are used for reforming, are supplied via the evaporator 62 to the reformer 71. In the warming-up of the fuel cell system, a three-way valve 76 is controlled so as to shut off the flow drawn into the fuel cell stack 60 until the warming-up of the reforming reactor 70 is completed.
If the HC content of the reformed gas supplied to the fuel cell stack 60 is high, the anode or a solid polymer membrane will be adversely effected, thereby causing a power reduction. Therefore, the completion of the warming-up of the system may be determined by determining whether the composition of the reformed gas has been stabilized, for example, whether the HC content has been stabilized at a specific level.
However, in the conventional method of warming up the fuel cell system, the water vapor generated by combustion using the burner 75 and the reformer 71 is condensed inside the “cold” reforming reactor 70 before the completion of the warming-up, so that waterdrops are produced and remain inside the system. In addition, the HC component of the reformed gas is dissolved in the condensed water; therefore, the condensed water has a high HC content. As the warming-up progresses, the condensed water is again evaporated and the evaporated component is diffused in the reformed gas; thus, the reforming reactor 70 is further warmed up after the dew point, and the HC content of the reformed gas does not stabilize until the condensed water is completely evaporated and disappears. Therefore, this system has the problem that a long time is required to complete the warming-up process.
In order to avoid water-vapor generation due to the burner 75, the system shown in FIG. 6, in which the burner 75 is omitted, may be used. In the warming-up when the system is started, the amount of air supplied to the evaporator 62 is larger than that supplied during the normal operation, and a large amount of the high-temperature air heated by the evaporator 62 is supplied to the reformer 71, thereby warming up the fuel cell system.
However, also in this method, the water vapor in the air is condensed in the reforming reactor 70, which has not yet been fully warmed up. Therefore, this system also has the problem explained above. In addition, this system employs no burner (75) and the warming-up is performed by only using heated air. In this case, the quantity of the supplied heat is smaller in comparison with the system having the burner. Therefore, a longer time is required before the warming-up is completed.